Apparatus including a scanning disk for transmitting television



y D. c. ESPLEY ET AL 2,206,546

APPARATUS INCLUDING A SCANNING DISK FOR TRANSMITTING TELEVISION FiledSept. 16, 195 3- Sheeis-Sheet 1 5| }-I 6 gp f j I 2 FEfi 5 ?L. I W W L Aw al--i 4 y 1 D. C. ESPLEY ET AL 7 2,306,545

APPARATUS INCLUDING A SCANNING DISK FOB TRANSMITTING TELEVISION FiledSept. 16, l937 351186125-511981 2 h f 4 a] INVEKTORS C, ill/40.

y 1940- D. c. ESPLEY E1: AL

APPARATUS INCLUDING A SCANNING DISK FOR TRANSMITTING TELEVISION FiledSept. 16, 1957 3 Sheets-Sheet c1 kn mij J, c1 0 2 u 1 u n D I U )2 a: Bif 4 92/? J n 0 5 """i u u n n n u u IIVVENTORS, Em C Patented July 2,1940 UNITED STATES PATENT OFFICE Dennis Clark Espley, North Wembley, andDerek Oscar Walter, Pinner, England, assignors to The General London,England Electric Company Limited,

Application September 16, 1937, Serial No. 164,168 In Great BritainSeptember 22, 1936 25 Claims.

This invention relates to television or like transmitters of the typewherein either an image of the picture to be transmitted is formed on aNipkow disk, by means of optical projection apparatus, or an image ofholes in a Nipkow disk is formed on the picture to be transmitted, theimage of the picture (or the picture itself) being scanned by the holes(or their image), respectively, as the case may be.

It is well known that, when the signals from such transmitters arereceived by a cathode ray tube, a characteristic form of distortion,known as keystone distortion, arises in the following way. During asingle picture frame period, during which a single picture isreproduced, the spot, controlled by equally spaced line-frequencysynchronising signals, traverses the cathode-ray screen M times in aseries of M parallel lines, all equidistant, of equal length, traversedin equal times, forming together a substantially rectangular area. Eachof these lines corresponds with the traverse of a single hole in theNipkow disk across the image. If therefore, the picture on the screen isto reproduce accurately the image on the disk, the holes must traversethe image in a series of straight lines all equidistant, of equallength, and traversed in equal times. The paths of the holes can be solittle curved that they can be taken as straight. Equidistance of thesepaths can be secured (it will appear later that it is not alwaysdesirable); but if, as is implied by the term Nipkow disk, the holes onthe disk are arranged on a spiral and not on a circle, the combinationof equal lengths with traverse in equal times cannot be attained. Forthe holes nearer the centre will travel slower than those more distant.

A precisely similar kind of distortion arises with other types ofreceiving apparatus (such as those using a mirror drum) which useconstant velocity scanning for the reconstruction of the picture.

If the image on the disk is still and rectangular, and if the scanningis plain (i. e. not interlaced), the result is a true keystonedistortion. (For brevity we shall suppose that the top line of thepicture on the cathode ray screen corresponds with the outermost hole onthe disk.) If the speeds of the disk and of the scanning spot areadjusted, so that the time of traverse of a spot across the screen isequal to the time of traverse of the innermost hole of the disk acrossthe image, then the bottom line of the picture will occupy exactly thefull width of the rectangular area on the screen. But then the time oftraverse of the outermost hole will be less than the time of traverse ofthe spot, and the top line of the picture will occupy part only of thatwidth. If the rectangular image on the disc is framed by a black border,the image on the screen will taper r If the original picture Wererectangular, it might be projected on the disk unaltered in shape andthe image masked appropriately. This simple procedure, although (as willappear later) it has some applications, would not be a real cure; forthough the outline of the picture on the screen would be rectangular,the lower lines would be reproduced on a larger scale than the upper. Analternative method, proposed in the specification of British LettersPatent No. 464,831 and producing a real cure, is to change therectangular picture into a keystone-shaped image in the course ofprojection, for example by reflections on surfaces inclined to theoptical axis of the projector. But such reflections are apt to introduceaberrations which militate against perfect definition. r

A far more serious distortion arises from the same cause when the imageis that of a moving film and the scanning is interlaced. It isunnecessary here to enter into the full theory of the interlacedscanning of moving films; it will be sufiicient to point out therelevant characteristics of one well-known form of it, which isdescribed and claimed in the specification of British Letters Patent No.465,184. A description is also to be found in the specification ofBritish Letters Patent No. 466,051.

In this form the centre line of the image of the film moves outwardalong a line through the centre of the disk at the same speed as thescanning hole moves inward, the radial distance between the outermostand innermost holes being equal to the length of a single picture in theimage. The number of scanning holes is odd, say 2N+1; the holes arearranged on P turns of a spiral; P may be any integer from 1 upwards,but is usually 2 or 4; if P is greater than 1 (as is prescribed in thesaid specification No. 465,184) some device (for example an interceptordisk) is provided to secure that at any given moment the holes in oneturn only are effective. (Itis possible in principle that P should benonintegral, but it is unnecessary to discuss this possibility indetail, since the value of P is not disk. The consequence important forour pur pose is that the odd numbered lines are ,all. scanned by holesnearer the centre oftheidisk while the even numbered lines lying betweenthem are all scanned by holes further irom the centre. Morespecifically, the (2n'+;l) thline (71. 0 to N) is scanned by the(N-l-n-i-Dth hole, while the 2nth line (n=1 to N) is scanned by the nthhole.

simple keystone distortion, but the even number lines are all shorterthan the odd number lines between which they-lie.

It is an object of the presentinvention to provide an improved methodand means for eliminating the keystone distortion arising from thevarying speed of the scanning holes of. a Nipkow disk.

A further object of the present invention is to provide means foreliminating the distortion caused by the varying speed of the scanningholes of a Nipkow disk in a television system transmitting an image of amoving film using interlaced scanning.

Other objects of this invention will appear during the course of thefollowing description and drawings in which:

Figure 1 and 2 show diagrammatically, and for the purpose ofexplanation, the nature of the distortion produced by an interlacedscanning of a moving film, Figures 3, 4 and 5illustrate-diagrammatically the removal of this distortion by means ofthe present invention.

Figure 6 shows diagrammatically a side elevation of an optical projectorutilizing the invention.

Figure 7 shows a side elevation of a rotatable disk for use with theprojector shown in Figure 6.

Figure 8 shows a cross-sectional view of the disk shown in Figure '7. 1

Figure 9 shows for the purpose of explanation one form of a mask. s

Figure 10 shows an improved mask.

Figure 11 shows a portion of a cinematograph film.

Figure 12 shows an alternative form of mask.

Figure 13 shows an explanatory diagram illustrating the derivation ofthe mask shown in Figure 12, and

Figure 14 is a cross-section on a reduced scale of the mask shown inFigure 12.

Figures 1 and 2 are explanatory diagrams showing how distortion occursin the above described system of transmitting an image of a moving filmwhen using interlaced scanning. Here, and in other figures to bedescribed later, N is taken as 6, and the difference in the length ofthe lines is greatly exaggerated; it would be impossible to show thetrue result clearly in a drawing of prac-' ticable size. In Figure 1,there is shown the image on the screen at a particular instant, and thepositions on it of the 2N+1 line's, numbered on the left, that have tobe reproduced on the screen. In Figure 2 there is shown'how these linesappear on the screen not, of course, simultaneously, but in succession.The numbers on the right are those of the holes by which the lines arescanned. The order in time of appearance is not that of either set ofnumbers; the order in which the lines of Figure 1 appear on the screenis 1, 3, 5, '7, 9, 11, 13, 2, 4, 6, 8, 10, 12. There is no necessaryrelation between the length of the lines in Figure 2 and the length ofthe lines in Figure 1; relative lengths within Figure 1 or within Figure2 alone matter.

It will be seen that the odd and even lines each 1 form on the screen apicture subject to simple keystone distortion; but the even lines areall shorter than the odd lines between which they lie. Let 0! be, thedistance of the outermost hole from the centre of the disk, 6 the radialdistance between consecutive holes, )\=1N6/d. N6/d will 7 be much lessthan 1, probably about 0.025. Then, The result is that the odd and evennumber lines'form on the screen a picture subject to with suflicientaccuracy, the ratio of the length of the top line in Figure 2 to thelength of the bottom line will be A, and so will be the ratio of aneven-numbered line to the pair of odd-numbered lines between which itlies. (For the purpose of the drawing Na/d has been taken as /2.)

If the image had been still and the scanning interlaced, the ratio ofthe length of the top to that of the bottom line would have beenAccordingly the interlaced scanning halves the general keystonedistortion, but it introduces a difference between successive lines thatis far more objectionable.

This distortion, which is due, as previously mentioned, to the varyingspeed of the holes in the scanning disk, cannot be abolished by anypracticable masking but is obviated by the present invention whichbroadly consists in providing means in a television or like transmitterof the type specified, whereby the magnification of the opticalprojection means is varied in time so that the magnification is lesswhen the image is being scanned by holes nearer the centre of the diskthan when it is being scanned by holes further from the centre of thedisk. Magnification. means, as usual, the ratio of a linear dimension ofthe object; of course this ratio may be, and usually will be, lessthan 1. The use of the term magnification implies that the ratio issubstantially constant over the image, which is therefore undistorted.

In regard to the principle of the present invention, the followingpoints should be understood. Let the holes in the disk be numbered byconsecutive integers 1' from the outermost hole inwards so that thedistance from the centre decreases as r increases. Then, in order thatthe principle described above should be used, and in order that theapparatus should fall within the invention, it is necessary that whensomehole r is scanning, the magnification is greater than when any hole(r-l-s) is scanning, and not greater than when any hole (TS) isscanning, s being positive. It is not necessary that the magnificationshould be greater when every hole 1' is scanning than when any hole(r+s) is scanning. When interlaced scanning is employed, some furthercondition must usually be imposed, dependent upon the nature of theinterlacing, and the extent to which keystone distortion is to beavoided. Thus, for the interlaced system discussed above, a suitablefurther condition to impose is that the magnification is the same forall values of r from 1 to N, and the same for all values of r from N+lto 2N+ 1, but is greater for the former values.

The application of the invention to an interlaced scanning system of thekind described above will now be described with reference to Figures 3,4 and 5 of the accompanying drawings.

In accordance with our invention, a television or like transmitter ofthe type specified which is provided with a Nipkow disk arranged toproduce interlaced scanning on a continuously moving film is providedwith an optical projection system whose magnification undergoes atemporal variation as the Nipkow disk rotates. The speed with which thefilm moves may be either constant or variable, provided that the film isnot brought to rest for an appreciable proportion of the period duringwhich each picture is being scanned. Transmitters in accordance with thepresent invention are accordingly distinguished from transmitters usingpreviously known methods of correcting for the effects of keystonedistortion in that while in the previously known methods themagnification of the image-producing means remained invariable in time(although, in some cases, being different at different points of theimage), in the present arrangement the magnification of theimage-producing means is varied from time to time during the rotation ofthe Nipkow disksuch variation being effected either continuously or atsuitable instants at which the Nipkow disk is in the appropriateposition or positions.

Figures 3 and 4 show, respectively, the image on the screen at twodifferent instants; Figure 3 refers to an instant at which the image isbeing scanned by one of the holes N+l to 2N+l; Figure 4 refers to aninstant at which it is being scanned by one of the holes 1 to N. Betweenthese two instants the magnification of the optical projection apparatushas been changed, so that the image Figure 3, is smaller than the imageFigure 4 in the ration x. The lines which will be scanned when the imagehas the size of Figure 3 (or Figure 4) are marked on the left of saidfigures. The picture on the screen will appear as shown in Figure 5.Since the even numbered lines in Figure 4 are longer in the ratio l/Athan the odd. numbered lines in Figure 3, the length of theeven-numbered lines in Figure 5 will be greater in that ratio, relativeto the length of the oddnumbered lines, than they were in Figure 2.There will be no longer a difference between alternate lines; but thewhole picture will be subject to simple keystone distortion in the ratio7i.

Instead of changing the magnification of the optical systemdiscontinuously from one fixed value to another as just described, themagnification could be changed continuously between the two extremevalues. With this arrangement, in an interlaced scanning system, theresulting picture would be produced free from both the distortion due tothe difference in length of alternate lines and the remaining simplekeystone distortion, while in the case of a non-interlacing scanningsystem, the resulting simple keystone distortion could in the same way,be abolished.

The required change in magnification, which must be eifected without anysubstantial change in the position of the image plane, is obtained bymeans of a known kind of compound lens in which an effectivedisplacement of one component thereof produces a change in magnificationwithout any substantial change in the position of the image planecorresponding with a given object plane.

The design of lenses having the properties just stated forms no part ofthe invention. It is sufficient therefore to record that such lenses canbe obtained from Messrs. Taylor, Taylor 8; I-Iobson of Leicester. Ofcourse, it is not implied that they cannot be obtained elsewhere.

Since the changes of magnification have to be made with extreme rapidity(e. g. 50 times a second), actual displacements of the components wouldbe inconvenient; but displacement can be simulated by change of therefractive index of the medium between the components. The refractiveindex might be changed by using as the medium, one of the known kind,the index of which is varied by an applied electric field, but a simplermethod is to introduce a glass plate between the components. Since glassis of higher refractive index than air, the introduction of a glassplate on one side of the said component, accompanied by removal oi" aglass plate from the other side of the said component, can be madeequivalent, to a first approximation, to displaging the said componenttowards said first pla e.

It will be understood that transparent materials other than glass may beused if desired, it being only necessary, in principle, that thematerial used should have a refractive index different from the air orother medium in which the lens components are immersed; and where theterm glass is used in the claims appended hereto, that term is to beunderstood to comprehend other suitable transparent materials having arefractive index different from. that of the medium in which the lenssystem is immersed.

One way of introducing and removing the glass plates is to mount twosemi-circular plates on the same shaft, so that (1) the plane of onelies on one side of the said component, and the plane of the other onthe other side, both these planes being perpendicular to the axis of theshaft, (2)

the bounding diameters of both plates are coplanar with the said axiswhich bisects both of them, (3) the two plates are on opposite sides ofthe plane mentioned under (2). When the shaft is rotated one or otherwill always intersect the axis of the lens, but never both. The speed ofrotation of the disc must be speed of rotation of the scanning disk R Pso that the position of the plates is restored at the instant when anygiven hole scans again. Since the introduction of one plate and theremoval of the other has to be completed during the short period(usually occupied by the black margin of the picture, between the twointerlaced scans, the circumference of the plates must be much greaterthan the diameter of the light beam where itpasses through the plates;but, as the example given below will show, the size of the platesrequired is not prohibitive.

It is obvious that more complicated arrangements might be used. Theplates might be segments of a circle including any angle l/p, where p isintegral. Then p of them have to lie in each plane on one side or theother of the said component, the angular distance between these adjacentedges being always /p; and the spaces in one plane have to be oppositethe plates in the other plane, that is to say, any line parallel to theaxis of rotation and intersecting the cross section of the beam of lighthas to intersect a plate in one plane and pass between two 75 plates inthe other. The speed of rotation must then be speed of rotation of thescanning disk Again the essential feature of introducing (or removing) aplate is the increase (or decrease) of the thickness of glass in thepath of the beam; it is not essential that this thickness should bereduced to zero. Accordingly, a circular plate of any constant thicknessmight be associated with the plate(s) aforesaid which lie in one plane;in other words, the plate(s) might be thicker portions of a continuouscircular disk.

Lastly, it will usually be necessary for mechanical reasons to supportthe glass plates at their adjacent edges by opaque metal, so that thereis a portion of the plane in which they lie which is neither glass platenor transparent space between glass plates. These opaque portions willbe arranged to cross the beam of light during the said short periodoccupied by the black margin. Accordingly the segments constituted bythe glass plates will not be exactly of angle 180/p, but will besubstantially 180), so that outside this short period, the effect is thesame as if the angle were 180).

Apparatus adapted to produce a change in magnification, in the mannerdescribed above, will now be described, by way of example, withreference to Figures 6, 7 and 8 of the accompanying drawings.

Figure 6 shows the general arrangement of the apparatus. 2| is a casingsupporting the coinponents of the lens. The outermost components 22 and23 are fixed. in the sides of the casing, the central component 24 issupported on a pedestal 25 within it. Above all these components theshaft 25 passes through bearings 21, 28 in the sides of the casing; itis driven by a synchronous motor. The glass plates are supported on thisshaft so as to lie in the planes 29, 30. The plane of the film is shownat 3!, that of the image on the disk at 32. The distances marked on thefigure have the following values:

a=102 mm. b: 81 mm. c: 41 mm. d: 29 mm. e: 88 mm. 1: 66 mm. g: mm.

is the maximum width of the beam of light. The components of the lensare such that the mean magnification is 1/ 1.5; it could be varied by2.5% by displacing the central component 24 by 239mm. along the opticalaxis.

Figure 7 shows in plan, and Figure 8 shows in section, the constructionof the disks which rotate with the shaft 26 in the planes 29, 30. Bothdisks are similar. 10 aforesaid is 1, so that the disks are displacedrelatively to each other by rotation through about the shaft, as well asby translation from 29 to 30 (or vice versa) along it.

Each disk consists of a steel frame 33 which is milled out of a solidplate. 34 is a central hole to take the shaft 26; 35 a hole which is thespace in the aforesaid description; 36 is a flanged hole into which theglass plate 31 fits and is held by tongues 38. It will be observed thatthe disk is asymmetrical about the diameter which separates the glassfrom the space, in order that it may be approximately balanced when theglass is inserted. Adjustable means of the usual kind for finalbalancing (not shown) are provided.

The distances shown on Figure '7 have the following values:

g: 120 mm. 5 h=2.649 ins. y'=2.993 ins. Ic=6.722 ins. 1:6.500 ins.m=0.250 ins. n=3.031 ins. r=6.142 ins.

The glass is 6.98 mm. thick; its refractive index for light ofwave-length 7500 A. is 1.5222. 15 It is to be observed that since therefractive index varies with the wave-length, it is not usually possibleto arrange that the magnification is the same for all wave-lengths, sothat the system is achromatic. The wave-length for which adjustment ismade is that at which the photoelectric cells to be used with theapparatus are most sensitive; their cathodes are, as usual, of thecaesium-oxygen-silver type. A red filter, transmitting preferentiallythe light to which the cell is most sensitive, may be inserted in thepath of the light to eliminate light of wave-length very different fromthat for which the adjustment is made.

However there is a complication that has been ignored. In order tosubstitute the picture of Figure 5 for the picture of Figure 2, it isnecessary to vary only the circumferential magnification, that is tosay, the magnification parallel to the tangent of the circle traversedby a hole at the point where it is midway between the two sides of theimage at which the hole enters and leaves the image. The methods justproposed for varying this magnification would usually vary in the sameratio the radial magnification, that 40 is to say, the magnificationparallel to the radius of the said circle at the said point. But if theradial magnification is reduced when the picture is being scanned by theholes nearer the centre, and if the pitch of the spiral on which theholes are arranged is the same both near the centre and remote from it,the lines produced in the received picture by the holes nearer thecentre will be further apart than those produced by the holes furtherfrom the centre. Accordingly the lines produced by one set of holes willnot fit uniformly between the lines produced by the other set of holes,as they must if correct interlaced scanning is to be obtained.

This change in the position of the lines perpendicular to their lengthisnot shown in Figure 5. Subsidiary means must be provided forcompensating the variation of the radial magnification, but not thevariation of the circumferential magnification, so that Figure 5represents the result accurately.

The simplest and preferred subsidiary means is a variation of the pitchof the spiral on which the scanning holes lie. Thus, if themagnification is reduced in the ratio A when the change 5 takes placebetween the scanning of the even lines by the outer holes and thescanning of the odd lines by the inner holes, then the pitch of thespiral (and therefore the radial distance between successive holes) mustbe reduced at the qo same time in the ratio A. This reduction can, ofcourse, be achieved by spacing the holes suitably when the disk is made.

Alternatively the subsidiary means might vary the speed with which theimage travels over the 75 disk. The condition that has to be fulfilled,in order that the variation of radial magnification shall becompensated, is that the time occupied by the image in traversing theradial distance between two successive holes should remain unchangedwhen the magnification is changed. -Reduction of magnification involvesa reduction in the same ratio, of the speed with which the image travelsradially over the disk. Reduction of the radial spacing between theholes, as aforesaid, reduces the distance to be travelled in the sameratio, and therefore keeps constant the time in which thedistance istravelled. But it could also be kept constant by increasing again thespeed with which the image travels, so as to abolish the reduction ofspeed associated with reduced magnification, the radial spacing of theholes remaining constant. This compensating increase of speed can beeffected by increasing the speed with which the film, the image of whichis projected, travels through the projecting apparatus past the gate,for example by suitable cams and other devices, obvious to those skilledin the art, in the driving mechanism. But, owing to the rapid changes ofspeed necessary, this method is not likely to be suitable except inspecial circumstances.

Lastly, subsidiary optical means might be employed which actuallyprevent change of the radial magnification. Thus, if a suitablecylindrical lens, with its axis parallel to the said tangent, wereintroduced into the path of the projecting beam at a suitable point atthe moment when the circumferential and radial magnifications of themain optical system are changed, it could (in principle at least)produce a change in the radial magnification opposing the change in theradial magnification of the main system. If the magnification of themain system is changed (as suggested above) by the introduction of aglass plate between the components of the main projecting lens, thiscylindrical lens might possibly be combined with the plate the part ofthe plate which intervenes in the optical path could be made acylindrical lens of suitable curvature with a circular axis coincidingwith the circle along which the said part of the plate travels.

But even now two forms of distortion remain. One is the simple keystonedistortion shown in Figure 5. This can usually be removed adequately bya mask as indicated above. If the mask were immediately before thesurface of the disk, it could be of the simple form shown in Figure 9 ofthe accompanying drawings. Here 39 is the disk, 49 the mask, exposingthe trapezoidal area 4! of the disk. 42 is the rectangular image thrownon the mask. The sides 43, dd are radii of the disk; the ends 55 and 46are so far apart that all the scanning holes pass between them. Nowevery hole takes the same time to cross the area 4!, so that there is noinclination of the vertical edges of the repro-- duced picture; at thesame time, the advantage is obtained of a border of constant blackness.But the part of the picture that falls on the mask is sacrificed;moreover, as said before the lower lines are reproduced on a largerscale than the upper. However these defects will often be negligible.

Generally it is more convenient to place the mask, not at the disk, butat the gate over which the film passes, and indeed to make the mask thegate. But now it must be remembered that, while they are being scanned,the two halves of the picture will be magnified differently; the masksmust be adjusted so that the two halves, each subjected to theappropriate magnification, combine to form on the disk an image such asis shown in Figure 9 aforesaid. The mask at the gate will thus take theform shown in Figure 10 of the accompanying drawings. It consists of twosuperimposed trapeziums il, it, 49, 50 and 5|, 52, 53, 54 which are suchthat the first (or second), if increased (or decreased)circumferentially in the ratio 1/). (or w), assumes the form 47, 48',52, 5|, (or 50, 49, 53254) so that the effective aperture of the mask,G1, 48', 53, fit (or 47, 33, 53', 54) is similar to the outline of thearea 4| in Figure 9.

An alternative form of gate and mask will now be described withreference to Figures 11-14.

The pictures proper on a moving film each of length are separated by astrip of length :0, usually black and hereinafter called black, bearingno picture; this arrangement is shown in- Figure 11 of the accompanyingdrawings. The pictures that have been considered above, scanned in 2N+1lines are necessarily coterminous; they therefore include not only thepicture proper, but a strip of black, 11/2 in length, on either side ofit. This strip should appear on the screen as dead black.

An examination of the formula given above, or of the numbers on theright of b in Figure 2, shows that the holes which scan these blackstrips are those numbered 1 to q, (N+1-q) to (N-i-l-l-q), and (2N+1-q)to (2N+1), where q is an integer near (i. e. differing by not as much as2 from) (2N+l) ac/iI-I. They are thus holes either at the two ends ofthe spiral or at its middle. The only function of these holes is then toproduce a dead black border for the picture in thescreen; they will dothis most eiciently if they are completely blocked; for then black willbe produced whatever the density of the black on the film. They mayactually be blocked on the disk; but, at least so far as the 7 middleholes are concerned, it is preferable to block them efiectively byobstructing matter in the gate, the image of which is projected on thedisk. The gate may therefore be shaped as shown in Figure 12 of theaccompanying drawings. It is derived from the gate shown in Figure 13 ofthe accompanying drawings, supposed of height H, by reducing that heightby a strip x/4 at either end and by placing abar of height 00/2 acrossthe. middle. This bar fulfills another very important function, namely,that of keeping the film flat on the gate and preventing irregularitiesin its motion, which are particularly serious when the scanning isinterlaced; indeed this will often be its main rather than a subsidiarypurpose. Of course, the bar could be introduced equally if the gate wererectangular,

and there were no masking to reduce keystone distortion. Its use ispossible-whenever, in. vir tue of interlaced scanning, there are holesin the disk, intermediate between the ends of the spiral, which neverscan any part of the picture except the black margins.

It .is usual to bevel the edges of a gate so as to avoid a penumbra onthe film. It is also desir-' able to bevel the edges of the'bar; thebevelling needs careful design because light has to reach the film nearthe edge from both sides (and not only one side) of the edge; but witha'suitable projecting system a penumbra can always be avoided. Asuitable formof bevellingis indicated in Figure 14, which showsdiagrammatically the crosssection through the gate shown in Figure 12.The film 55 travels over the face of the gate member 515 which carriesthe bar 51; and in order to assist in keeping the film 55 in place onthe surface of the gate member 56, a pressure plate 56' is provided,this plate 56' carrying a bar 51. As will be seen in the drawing, theedges 58, 59, 58', 59, of the gate apertures, and the edges 6! 6|, 60,SI, of the bars 51 and 51' are likewise bevelled, so that they do notinterfere with the light passing through the gate apertures.

An alternative method of removing the keystone distortion shown inFigure 5 would be to change the magnification of the optical apparatuscontinuously between the two extremes and not discontinuously. Thus if,while the last (N+1) holes were scanning, the magnification decreasedlinearly with time from m (say) to Mn; if it changed discontinuously tom/x during the change from the (2N+1)th to the 1st hole; and if, whilethe first N holes were scanning, it decreased linearly with time fromm/A to m; then, as an examination of Figure 2 will show, a picture wouldbe produced on the screen free from both keystone distortion and from adifference between alternative lines. Such changes might be produced bytapering suitably the thickness of the glass plates, mentioned above indescribing the preferred method of varying the magnification. But,unless the diameter of the lens is very small compared with the lengthof the circle on the plates passing through the lens, the tapering platewill act as a prism displacing the image appreciably.

But yet another form of distortion remains, which affects not the shapeof the reproduced picture, but the distribution of illumination in it;the ratio of the mean brightness of the part of the reproduced picturescanned by the holes nearer the centre to the mean brightness of thepart scanned by the holes further from the centre will not be the sameas the ratio of brightnesses of the corresponding parts in the originalpicture. For the change of magnification will produce a change ofbrightness in the resulting image, the brightness usually increasing asthe magnification decreases. All these differences of bright- I ness inthe image will produce corresponding differences of lightness in thepicture on the screen. Accordingly in Figure 5 the odd-numbered lines,since they result from the scanning of an image of small magnification,will usually be brighter than the even-numbered lines. Means aretherefore preferably introduced for correcting this brightnessdistortion.

One method always available for the correction of brightness distortionis that mentioned in the specification of British Letters Patent No.466,780, namely, the introduction into the projecting beam, where animage is formed, of material of which the absorption for light variesover the image, so that it is great at those parts of the image which,in the absence of the material, would correspond with abnormally brightparts of the reproduced image and is small at those corresponding withabnormally dull parts. This method usually requires the formation of animage in some plane other than that of the disk. This plane, at whichthe material is placed, preferably lies between the source of light andthe disk, but might lie between the disk and the photoelectric cell orother receiver.

In a second method, the absorption in the brightness is relativelygreat.

path of the projection beam'is varied, 'not in space, but in time. Thuswhen that part of the image is being scanned which is apt to be toobright in the reproduced picture, absorbing matter is introduced intothe beam, when the parts apt to be too dull are being scanned, theabsorbing matter is removed. This method is most suitable in connectionwith the aforesaid preferred method of varying the magnification, solong as at least one glass plate is present between the components ofthe projecting lens when the For this plate can be caused to absorb thelight sufficiently to reduce the intensity of the projecting beam in thedesired ratio. The term absorb" is to be interpreted widely; it includesloss of light in the beam due to reflection, as well as that due to trueabsorption.

In the third method, possible in principle, but likely to be diflicultto apply in practice, the intensity of the beam is varied in time, notby introducing absorbing matter, but by varying the output of thesource. If the disk is driven synchronously with the alternations of themain supply, the source of light may be a lamp fed by the same supplyand the flicker, characteristic to a greater or less extent of all lampsrun on A. C., may be used to compensate the brightness distortion.

In a fourth, and generally preferable method, the brightness distortionis compensated by a variation of the area of the holes in the disk.These holes which, if the area of all holes were the same, would producetoo low an illumination in the reproduced picture may be made slightlylarger than the others. The advantage of this method of compensation isthat it can be used however complicated the brightness distortion maybe. For difierences in brightness will always be associated withdifferent holes and can therefore be compensated by variations in theholes. Further, the method lends itself most readily to an adjustment ofthe compensation by trial, rather than by calculation, which will almostalways be necessary.

In yet a fifth method, the brightness distortion itself is leftunaltered and the correction is applied to the electrical signalsproduced by the light when they act on the photoelectric cell or otherreceiver. Thus the primary current through the photoelectric cell willalways be amplified, for example, in amplifying valves or in secondaryemission electron multipliers; it will be obvious to those skilled inthe art that the gain of thesestages may be varied so as to compensatefor brightness distortion, and so that the signals finally emitted fromthe transmitters are free from this distortion. Further, if (as isusual) the television transmitting apparatus generates electricalsynchronising signals determined by the passage of the individual holesacross the image, it will be obvious how these signals may be used tovary the gain in the appropriate manner.

Of course any of these methods may be used in conjunction with any ofthe others.

We claim:

1. A television or like transmitter of the type specified including aNipkow disk having scanning apertures suitably disposed to produce aninterlaced scan on a continuously moving film, an optical projectionsystem associated with said disk and means for varying temporally themagnification of said optical projection system with reference to theradial distance, from the centre of said disk. of the hole by whichscanning is a compound lens of the kind in which an effectivedisplacement of one component changes the magnification of said compoundlens, but does not change substantially the position of the image planecorresponding with a given object plane, and means for producing fromtime to time an effective displacement of said component with referenceto the radial distance, from the centre of said disk, of the hole bywhich scanning is being performed, whereby the magnification of saidoptical projection system is reduced as the radial distance of said holedecreases.

3. In a television or like transmitter of the type specified a Nipkowdisk having scanning apertures suitably disposed to produce aninterlaced scan on a continuously moving film, an optical projectionsystem including a compound lens associated with said disk, saidcompound lens having a component, the efiective movement of whichchanges the magnification of said lens without changing substantiallythe position of the image plane corresponding with a given object plane,and means for varying the refractive index of the media on either sideof said component with reference to the radial distance, from the centreof said disk, of the hole by which scanning is being performed, wherebythe magnification of said optical system is reduced as the radialdistance of the said hole decreases.

l. In a television or like transmitter of the type specified including aNipkow disk having at least two series of scanning holes, said secondseries of holes being nearer the centre of said disk than said firstseries of holes, means for correcting keystone distortion comprising anoptical system including a compound lens for projecting an image of thepicture to be transmitted on said disk, said compound lens having acomponent, the effective movement of which changes the magnification ofsaid lens without changing substantially the position of the image planecorresponding with a given object plane, and means for temporallyvarying the magnification of said optical projection system with thevariation of the radial distance from the centre of said disk to anoperative scanning hole, said means comprising a pair of glass sheetsdisposed in planes on opposite sides of the plane of said component, andmeans to alternately introduce one of said glass sheets on one side ofsaid component when one of said series of holes is scanning and theother of said glass sheets on the other side of said component when theother of said series of holes is scanning.

5. In a television or like transmitter of the type specified a scanningdisk, an optical system including a compound lens for projecting animage of a moving film on said disk, said compound lens having acomponent, the effective ,movement of which changes the magnification ofsaid lens without changing substantially the position of the image planecorresponding with a given object plane, and means for changing fromtime to time the thickness of glass in two planes, one on either side ofthe said component,

the thickness of the glass in one of said planes being increased as thethickness of the glass in the other said plane is decreased.

6. In a television transmitter of the type specified, a scanning disk,means for correcting keystone distortion comprising an optical system,including a compound lens comprising three spaced components, forprojecting an image of an object onto said disk, a shaft rotatablymounted in bearings adjacent said compound lens, the axis of said shaftbeing parallel with the optical axis of said lens, means to rotate saidshaft, a pair of disks mounted on said shaft and lying in planes betweenthe central component and each of the outer components of said compoundlens, said disks supporting sector shaped glass plates, the projectionsof said sector-shaped plates upon each other being non-overlapping,whereby as said shaft is rotated said plates are alternately disposed onopposite sides of said central component.

'7. For a television system of the type specified, a casing, an opticalsystem comprising a compound lens, having three spaced components, ofthe kind in which the effective displacement of the central componentchanges the magnification of the lens without substantially changing theposition of the image plane corresponding with a given object plane, theouter components of said lens being supported by the walls of saidcasing and a pedestal mounted within said casing supporting the centralcomponent of said :lens, a shaft rotatably mounted in bearings in thewalls of said casing, the axis of said shaft being parallel with theoptical axis of said lens, means to rotate said shaft, two disksperpendicularly mounted on said shaft so as to intercept the opticalaxis of said lens, and situated one on either side of said centralcomponent, each of said disks carrying a segment of a circular glassdisk, the projections of said glass segments upon each other beingnon-overlapping.

8. The method of television transmission of moving picture film usinginterlaced scanning which comprises the steps of projecting said filmthrough an optical system to form an image of said film on a rotatingscanning disk having two series of holes, scanning the image with thefirst series of holes, reducing the width of said image by reducing themagnification of said optical system and thereafter scanning the reducedwidth image with said second series of holes.

9. The method of television transmission of moving picture films usinginterlaced scanning which comprises the steps of optically projecting animage of said film onto a rotating scanning disk having two series ofholes formed therein, whereof the radial distance between successiveholes in the second series is less than the radial distance betweensuccessive holes in the first series, scanning the image with the firstseries of holes, reducing the magnification of the opti-. cal projectionsystem and thereby the size of said image and thereafter scanning thereduced image with said second series of holes.

10. In a television transmitter of the type specified a scanning diskhaving scanning apertures suitably disposed to produce an interlacedscan on a continuously moving film, an optical projection systemincluding a compound lens whose magnification can be varied from time totime for forming an image of a picture to be transmitted on said disk,said scanning disk being formed with two series of scanning holes,

whereof the radial distance betweenlsuccessive holes in the secondseries is less than the radial distance between successive holes in thefirst series, and means for reducing the size of said image by reducingthe magnification of said optical projection system when said image isbeing scanned by said second series of holes.

11. In a television transmitter of the type specified a scanning diskhaving scanning apertures suitably disposed to produce an interlacedscan on a continuously moving film an optical projection systemincluding a compound lens whose magnification can be varied from time totime for forming an image of a picture to be transmitted on said disk,said scanning disk being formed with two series of scanning holes,whereof the radial distance between successive holes in the secondseries is less than the radial distance between successive holes in thefirst series, and the holes of said second series are of smaller areathan the holes of said first series, and means for reducing the size ofsaid image by reducing the magnification of said optical projectionsystem when said image is being scanned by said second series of holes.

12. In a television or like transmitter of the type specified, means forcorrecting keystone distortion comprising a Nipkow disk having at leasttwo series of scanning holes, said second series of holes being nearerthe centre of said disk than said first series of holes, an opticalsystem including a compound lens for projecting an image of the pictureto be transmitted on said disk, said compound lens having a component,the effective movement of which changes the magnification of said lenswithout changing substantially the position of the image planecorresponding with a given object plane, means for varying themagnification of said optical system including a pair of glass sheetsdisposed in planes on opposite sides of the plane of said component, andmeans to alternately introduce one of said sheets on one side of saidcomponent as one of said series of holes is scanning and the other ofsaid sheets on the other side of said component as the other of saidseries of holes is scanning whereby the magnification of said projectionsystem is reduced when said image is being scanned by said second seriesof holes, and means for preventing an alteration in the brightness owingto the change in magnification comprising forming said glass sheetintroduced into said optical system when said second series of holes isscanning a glass that has a greater absorption than has the glass of theother glass sheet.

13. In a television transmitter of the type specified, a scanning diskhaving scanning apertures suitably disposed to produce an interlacedscan on a continuously moving film an optical projection systemincluding a compound lens for forming an image of a moving film on saiddisk, means for varying the magnification of said optical system withreference to the radial distance from the centre of the'said disk of thehole by which scanning is being performed whereby distortion arisingsubstantially from the different speeds of different scanning holes isreduced or abolished and means for reducing the resulting simplekeystone distortion comprising a mask fixed relative to the projectingbeam and formed with an aperture in the shape of two superimposedtrapezia.

14. In a television transmitter of the type specified, a scanning diskhaving scanning apertures suitably disposed to produce an interlacedscan on a continuously moving film,. an optical projection systemincluding a compound lens for forming an image of a moving film on saiddisk, means for varying from time to time the magnification of saidoptical system with reference to the radial distance from the centre ofthe said disk of the hole by which scanning isbeing performed wherebydistortion arising substantially from the different speeds of differentscanning holes is reduced or abolished and means for reducing theresulting simple keystone distortion comprising a gate, over which saidfilm is drawn, formed with two apertures each of the shape of atrapezium separated by a bar of opaque material.

15. A television or like transmitter of the type specified including ascanning disk, an optical projection system associated with said diskand means for producing a periodic variation from time to time in themagnification of said optical system in dependence upon the distance ofthe operative scanning point of the disk from the centre of said disk,the variation in the magnification of said optical projection system(when effected) being such that the distance of the centre of the singlelens equivalent (for paraxial rays) to the said optical system from thescanning disk is reduced as the radial distance of said scanning pointdecreases.

16. A television or like transmitter of the type specified including ascanning disk, an optical tance of the operative scanning point of thedisk 1 from the centre of said disk, the variation in the magnificationof said optical projection system (when efiected) being such that thedistance of the centre of the single lens equivalent (for paraxial rays)to the said optical system from the scanning disk is reduced as theradial distance of said scanning point from the centre of said diskdecreases, the difference between the radii of neighboring scanningpoints being reduced when the aforesaid distance of the said centre fromthe scanning disk is reduced.

17. In a television transmitter of the type specified, a scanning diskhaving scanning apertures suitably disposed to produce an interlacedscan on a continuously moving film, an optical projection systemassociated with the said disk, means for periodically varying themagnification of said optical system from time to time in dependenceupon the distance of the operative scanning point of the disk from thecentre of said disk, the variation in the magnification of said opticalprojection system (when effected) being such that the distance of thecentre of a single lens equivalent (for paraxial rays) to the saidoptical system from the scanning disk is reduced as the radial distanceof said scanning point decreases, and means for masking the resultingsimple keystone distortion comprising a film gate formed with apertureseach of the shape of a trapezium separated by a bar of opaque material.

18. In a television transmitter of the type specified, a scanning diskhaving scanning apertures suitably disposed to produce an interlacedscan on a continuously moving film, an optical projection systemassociated with said disk, means for periodically varying themagnification of said optical projection system from time to time independence on the distance of the operative scanning point of the diskfrom the centre of said disk, the variation in the magnification of said7';

optical projection system (when effected) being such that the distanceof the centre of a single lens equivalent (for paraxial rays) to thesaid optical system from the scanning disk is reduced as the radialdistance of said scanning point from the centre of said disk decreases,the diiierence between the radii of neighbouring scanning points beingreduced when the aforesaid distance of the said centre from the scanningdisk is pr0- duced, and means for masking the resulting simple keystonedistortion comprising a film gate formed with apertures each of theshape of a trapezium separated by a bar of opaque material.

19. In a television or like transmitter of the type specified includinga scanning disk having scanning apertures suitably disposed to producean interlaced scan on a continuously moving film, an optical projectionsystem associated with said disk and means for periodically varying themagnification of said optical system from time to time in dependenceupon the distance of the operative scanning point of the disk from thecentre of said disk, the variation in the magnification of said opticalprojection system (when effected) being such that the distance of thecentre of the single lens equivalent (for paraxial rays) to the saidoptical system from the scanning disk is reduced as the radial distanceof said scanning point decreases, and means for varying the absorptionof the optical system when its magnification is varied.

20. In a television transmitter of the type specified, a scanning diskhaving scanning apertures suitably disposed to produce an interlacedscan on a continuously moving film, an optical projection systemassociated with said disk, means for periodically varying themagnification of said optical projection system from time to time independence upon the distance of the operative scanning point of the diskfrom the centre of said disk, the variation in the magnification of saidoptical projection system (when efiected) being such that the distanceof the centre of the single lens equivalent (for paraxial rays) to thesaid optical system from the scanning disk is reduced as the radialdistance of said scanning point from the centre of said disc decreases,the difference between the radii of neighbouring scanning points beingreduced when the aforesaid distance of the said centre from the scanningdisk is reduced, means for masking the resulting simple keystonedistortion comprising a film gate formed with apertures each of theshape of a trapezium separated by a bar of opaque material, and meansfor varying the absorption of the optical system when its magnificationis varied.

21. A television or like transmitter of the type specified including aNipkow disk having scanning apertures suitably disposed to produce aninterlaced scan on a continuously moving film, an optical projectionsystem associated with said disk and means for periodically varying {themagnification of said optical system from time to time in dependenceupon the distance of the operative scanning point of the disk from thecentre of said disk, the variation of said optical projection system(when effected) being such that the distance of the centre of the singlelens equivalent (for paraxial rays) to the said optical system from thescanning disk is reduced as the radial distance of said scanning pointdecreases, the diameter of holes more remote from the centre of saiddisk being greater than the diameter of a plurality of holes nearer tothe centre of said disk.

22. A television or like transmitter of the type specified including aNipkow disk having scanning apertures suitably disposed to produce aninterlaced scan on a continuously moving film, an optical projectionsystem associated with said disk, means for periodically varying themagnification of said optical projection system from time to time independence upon the distance of the operative scanning point of the diskfrom the centre of said disk, the variation in the magnification of saidoptical projection system (when cfiected) being such that the distanceof the centre of the single lens equivalent (for paraxial rays) to thesaid optical system from the scanning disk is reduced as the radialdistance of said scanning point from the centre of said disk decreases,the difierence between the radii of neigorbouring scanning points beingreduced when the aforesaid distance of the said centre from the scanningdisk is reduced, the diameter of holes more remote from the centre ofsaid disk being greater than the diameter of a plurality of holes nearerto the centre of said disk.

23. In a television transmitter of the type specified, a Nipkow diskhaving scanning apertures suitably disposed to produce an interlacedscan on a continuously moving film, an optical projection systemassociated with the said disk, means for periodically varying themagnification of said optical system from time to time in dependenceupon the distance of the operative scanning point of the disk from thecentre of said disk, the variation of the magnification of said opticalprojection system (when effected) being such that the distance of thecentre of the single lens equivalent (for paraxial rays) to the saidoptical system from the scanning disc is reduced as the radial distanceof said scanning point decreases, the diameter of holes more remote fromthe centre of said disk being greater than the diameter of a pluralityof holes nearer to the centre of said disk, and means for masking theresulting simple keystone distortion comprising a film gate formed withapertures each of the shape of a trapezium separated by a bar of opaquematerial.

24. In a television transmitter of the type specified, a Nipkow diskhaving scanning apertures suitably disposed to produce an interlacedscan on a continuously moving film, an optical projection systemassociated with said disk, means for periodically varying themagnification of said optical projection system from time to time independence upon the distance of the operative scanning point of the diskfrom the centre of said disk, the variation in the magnification of saidoptical projection system (when effected) being such that the distanceof the centre of a single lens equivalent (for paraxialrays) to the saidoptical system from the scanning disk is reduced as the radial distanceof said scanning point from the centre of said disk decreases, thedifierence between the radii of neighbouring scanning points beingreduced when the aforesaid distance of the said centre from the scanningdisk is reduced, the diameter of holes more remote from the centre ofsaid disk being greater than the diameter of a plurality of holes nearerto the centre of said disk, and means for masking the resulting simplekeystone distortion comprising a film gate formed with apertures each ofthe shape of a trapezium separated by a bar of opaque material.

. distortion, said means comprising an optical system for projectingsaid film on said disk and means for varying the magnification of saidoptical system, and means for correcting the resulting simple keystonedistortion.

DENNIS CLARK ESPLEY. 5 DEREK OSCAR WALTER.

